Cu precipitation strengthened steel and method for producing the same

ABSTRACT

The Cu precipitation strengthened steel of the invention comprises, on the mass percent basis, C: 0.02-0.10%, Mn: 0.3-2.5%, Cu: 0.50-2.0%, Ni: 0.3-4.0% and Ti: 0.004-0.03% and further comprises Si: 0.01-0.4% and/or Al: 0.001-0.1%, with the contents of incidental impurities being P: not more than 0.025%, S: not more than 0.01%, N: not more than 0.006% and Se: not more than 0.005%, with the value of Pcm defined by the formula (1) given below being not more than 0.28. The steel material made of this Cu precipitation strengthened steel has good and stable CTOD toughness and is suited for use as a steel material for the construction of large industrial machines, ships, marine structures, line pipes, tanks, bridges and like welded structures.  
             Pcm   =     C   +     Si   30     +     Mn   20     +     Cu   20     +     Ni   60     +     Cr   20     +     Mo   15     +     V   10     +     5      B               (   1   )                       
 
     In the above formula, C, Si, Mn, Cu, Ni, Cr, Mo, V and B respectively represent the contents (in mass percent) of the respective elements.

BACKGROUND

[0001] 1. Field of the Invention

[0002] The present invention relates to a Cu (copper) precipitationstrengthened steel suited for use as a material for the construction oflarge industrial machines, ships, marine structures, line pipes, tanks,bridges and like welded structures, and to a method of producing thesame.

[0003] 2. Description of the Related Art

[0004] Recent years have seen continuous increases in the strength ofwelded structures, such as large industrial machines, ships, marinestructures, line pipes, tanks and bridges, as requested from theviewpoint of economy and safety. To keep up with such trend, higher andhigher levels of characteristics have been required of steel products tobe used as materials for construction of these welded structures. One ofthe characteristics required of these steel products is the CTODtoughness, which is determined by the fracture toughness test accordingto BS 7448 or ASTM E 1290. Improvements in and stabilization of the CTODtoughness greatly contribute to the improvement in the safety of weldedstructures.

[0005] The “CTOD toughness” is an indicator of the resistance to CTOD(Crack Tip Opening Displacement). More specifically, a test specimengiven a fatigue crack is subjected to three point bending at a giventemperature and the opening displacement at the crack tip is measuredwith a clip gage or the like. The CTOD toughness is evaluated in termsof the critical value of crack tip opening displacement at the time offracture (hereinafter referred to as “critical CTOD value”).

[0006] It is known in the art that it is effective in improving the CTODtoughness of steel products to reduce the C content in the steel. Tocompensate for the decrease in strength as resulting from the reductionin C content in steel, various alloying elements are added and/or theproduction process is modified to increase the strength. Thus, forexample, steel products which utilize Cu precipitation hardening aredisclosed in ASTM A 710 and U.S. Pat. No. 3,692,514. These steelproducts are characterized by their being excellent in weldability.Improvements in their toughness in low temperature environments arestill desired, however.

[0007] While the CTOD toughness is evaluated by using a plurality oftest specimens collected from one steel product and carrying out aplurality of test runs under the same conditions, some test specimensmay show markedly lower critical CTOD values as compared with other testspecimens in certain instances even when they are tested under the sameconditions. The CTOD toughness required of steel products is evaluatedin terms of the lowest critical CTOD value (hereinafter referred to as“minimum critical CTOD value”) among the critical CTOD values of thosetest specimens tested under the same conditions and, therefore, it isnecessary that the minimum critical CTOD value for a steel productshould clear a given value. Therefore, in the art, the C content insteel is reduced to an excessive extent or expensive alloying elementsare added in large amounts to thereby excessively improve the CTODtoughness in preparation for the phenomenon mentioned above. As aresult, it has been difficult to reduce the cost of production of Cuprecipitation strengthened steels.

SUMMARY

[0008] Accordingly, it is an object of the present invention to providea Cu precipitation strengthened steel having good and stable CTODtoughness as well as a method of producing the same.

[0009] The Cu precipitation strengthened steel of the inventioncomprises, on the mass percent basis, C: 0.02-0.10%, Mn: 0.3-2.5%, Cu:0.50-2.0%, Ni: 0.3-4.0% and Ti: 0.004-0.03% and further comprises Si:0.01-0.4% and/or Al: 0.001-0.1%, with the contents of incidentalimpurities being P: not more than 0.025%, S: not more than 0.01%, N: notmore than 0.006% and Se: not more than 0.005%, with the value of Pcmdefined by the formula (1) given below being not more than 0.28:$\begin{matrix}{{Pcm} = {C + \frac{Si}{30} + \frac{Mn}{20} + \frac{Cu}{20} + \frac{Ni}{60} + \frac{Cr}{20} + \frac{Mo}{15} + \frac{V}{10} + {5B}}} & (1)\end{matrix}$

[0010] where C, Si, Mn, Cu, Ni, Cr, Mo, V and B respectively representthe contents (in mass percent) of the respective elements.

[0011] The strength can further be improved by causing the steel tocontain, on the mass percent basis, at least one element selected fromthe group consisting of Cr: 0.05-1.0%, Mo: 0.05-1.0%, Nb: 0.005-0.04%,V: 0.01-0.10% and B: 0.0005-0.003%.

[0012] The toughness can further be improved by causing the steel tocontain, on the mass percent basis, at least one element selected fromthe group consisting of Ca: 0.0005-0.05%, Zr: 0.0005-0.05% and REMs(rare earth metals): 0.0005-0.05%.

[0013] The steel of the present invention can be produced by aproduction process comprising the following steps (a) to (e), which isgiven as an embodiment of the present invention:

[0014] Step (a): Heating a steel having the above chemical compositionto a temperature not lower than 950° C. but not higher than 1250° C.;

[0015] Step (b): Hot rolling the thus-heated steel;

[0016] Step (c): Allowing the hot-rolled steel to cool or cooling thesame in an accelerated manner;

[0017] Step (d): Reheating the steel after being allowed to cool oracceleratedly cooled to a temperature not lower than 450° C. but nothigher than 680° C.; and

[0018] Step (e): Air cooling the reheated steel.

[0019] When it is intended that the stability of welded structures inwhich the steel product of the invention is applied as a material forconstructing them be improved, the steel can also be produced by anotherembodiment of the production process of the present invention whichcomprises the following steps (A) to (C):

[0020] Step (A): Estimating, for a steel having the above chemicalcomposition, the change in tensile strength in the process ofstrain-removing heat treatment on the assumption that the steel may besubjected to strain-removing heat treatment after tempering undervarious conditions;

[0021] Step (B): Determining the tempering conditions based on thechange in tensile strength as estimated in step (A); and

[0022] Step (C): Tempering the steel under the tempering conditionsestablished in step (B).

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 (a) is a graphic representation of a heating pattern to befollowed on the assumption that a Cu-containing steel as rolled issubjected to strain-removing heat treatment.

[0024]FIG. 1 (b) is a graphic representation of the results ofcalculations for estimating the change in tensile strength at ordinarytemperature as made for the same Cu-containing steel after applicationof the heating pattern shown in FIG. 1 (a).

[0025]FIG. 2 (a) is a graphic representation of a heating pattern to befollowed on the assumption that a Cu-containing steel is subjected totempering at 500° C. for 1 hour and strain-removing heat treatment.

[0026]FIG. 2 (b) is a graphic representation of the results ofcalculations for estimating the change in tensile strength at ordinarytemperature as made for the same Cu-containing steel after applicationof the heating pattern shown in FIG. 2 (a).

[0027]FIG. 3 (a) is a graphic representation of a heating pattern to befollowed on the assumption that a Cu-containing steel is subjected totempering at 550° C. for 1 hour and strain-removing heat treatment.

[0028]FIG. 3 (b) is a graphic representation of the results ofcalculations for estimating the change in tensile strength at ordinarytemperature as made for the same Cu-containing steel after applicationof the heating pattern shown in FIG. 3 (a).

[0029]FIG. 4 (a) is a graphic representation of a heating pattern to befollowed on the assumption that a Cu-containing steel is subjected totempering at 600° C. for 1 hour and strain-removing heat treatment.

[0030]FIG. 4 (b) is a graphic representation of the results ofcalculations for estimating the change in tensile strength at ordinarytemperature as made for the same Cu-containing steel after applicationof the heating pattern shown in FIG. 4 (a).

[0031]FIG. 5 (a) is a graphic representation of a heating pattern to befollowed on the assumption that a Cu-containing steel is subjected totempering at 650° C. for 1 hour and strain-removing heat treatment.

[0032]FIG. 5 (b) is a graphic representation of the results ofcalculations for estimating the change in tensile strength at ordinarytemperature as made for the same Cu-containing steel after applicationof the heating pattern shown in FIG. 5 (a).

DETAILED DESCRIPTION OF THE INVENTION

[0033] The steel of the present invention is a Cu precipitationstrengthened steel stabilized in CTOD toughness by reducing the contentof Se, which is an incidental impurity, to 0.005% or below to therebyinhibit the formation of inclusions of Se, based on the finding,obtained upon investigation concerning the phenomenon of markedly lowcritical CTOD values being sometimes found in the CTOD test, that thisphenomenon is caused by inclusions of the incidental impurity Se.

[0034] In the following, the Cu precipitation strengthened steel of thepresent invention and the method of producing the same are morespecifically described. In the following description, the percentageindicating the content of each chemical constituent means “% by mass”.

[0035] Chemical Composition of the Steel:

[0036] C: 0.02-0.10%

[0037] C is an element contributing toward increasing the strength. At alevel below 0.02%, the strength can hardly be secured. On the otherhand, at a content exceeding 0.10%, it deteriorates the desiredweldability and toughness of the product. Hence, the C content should benot less than 0.02% but not more than 0.10%. From the economy and highperformance viewpoint, the C content is desirably not less than 0.03%but not more than 0.08%.

[0038] Mn: 0.3-2.5%

[0039] Mn is an element necessary for securing the strength andtoughness of steel. At a level lower than 0.3%, such effects are slightwhile addition thereof at a high level exceeding 2.5% results indeterioration of the weldability. Hence the Mn content should be notless than 0.3% but not more than 2.5%. From the economy and highperformance viewpoint, the Mn content is desirably not less than 0.6%but not more than 1.8%.

[0040] Cu: 0.5-2.0%

[0041] Cu is an element characterizing the present invention. Byutilizing the Cu precipitation hardening, the C content can be reducedand thereby the weldability and toughness of steel can be improved. Forattaining effective Cu precipitation hardening, the Cu content should benot less than 0.5%. On the other hand, an excess Cu content converselyresults in decreases in toughness of steel and at the same time impairsthe hot workability of steel. Therefore, the Cu content should be nothigher than 2.0%. From the economy and high performance viewpoint, theCu content is desirably not less than 0.7% but not more than 1.8%.

[0042] Ni: 0.3-4.0%

[0043] Ni is effective in preventing the occurrence of cracks in thestep of hot working, which is the phenomenon intrinsic in Cu-containingsteels, to thereby improve the workability. It is also effective inimproving the strength of steel. For efficiently achieving theseeffects, the Ni content should be not less than 0.3%. For suppressingthe above-mentioned cracking in the step of hot working, it is desirablethat Ni be contained at a level not lower than half the Cu content. Onthe other hand, at an excessive Ni content, scale defects tend toappear. For avoiding this, the Ni content should be not more than 4.0%.Since Ni is an expensive element, the Ni content is desirably not morethan 2.0% from the economy viewpoint.

[0044] Ti: 0.004-0.03%

[0045] Ti is an element capable of fixating solute-form N, which impairsthe toughness of steel, and rendering the same harmless and at the sametime effective in inhibiting austenite grains from coarsening to therebyimprove the toughness of steel. For obtaining this effect, the Ticontent should be not less than 0.004%. On the other hand, an excessiveTi content conversely brings about decreases in toughness of steel.Since, according to the present invention, the N content should be notmore than 0.006%, as mentioned later herein, the Ti content should benot more than 0.03% in proportion to the N content. From the highperformance viewpoint, the Ti content is desirably not less than 0.005%but not more than 0.015%.

[0046] Si: 0.01-0.4%, Al: 0.001-0.1%

[0047] Si and Al both deoxidize steel and are thus effective inrendering the steel sound. At excessive levels, however, theydeteriorate the toughness and weldability of steel. Therefore, one orboth of them are caused to be contained, Si within the content range ofnot less than 0.01% but not more than 0.4% and Al within the contentrange of not less than 0.001% but not more than 0.1%. From the economyand high performance viewpoint, the Si content is desirably within therange of not less than 0.01% but not more than 0.2% and the Al contentis desirably within the range of not less than 0.001% but not more than0.04%. Si is also effective in increasing the strength of steel and,when it is contained in the above range, this effect can also beproduced.

[0048] P: not more than 0.025%, S: not more than 0.01%, N: not more than0.006%

[0049] P, S and N, which are incidental impurities, markedly reduce thetoughness of steel. Therefore, it is desirable to reduce the P, S and Ncontents to levels as low as possible. To markedly reduce the contentsof these elements, however, proportional costs are required. Therefore,in accordance with the invention, the P content is restricted to 0.025%or less, the S content to 0.01% or less and, the N content to 0.006% orless.

[0050] Se: not more than 0.005%

[0051] Se, which is an incidental impurity, forms hard and brittleinclusions in steel and thereby markedly decreases the CTOD toughness ofsteel. Therefore, the Se content should be not more than 0.005%. For thehigher performance viewpoint, the Se content is desirably not more than0.001%.

[0052] Cr: 0.05-1.0%, Mo: 0.05-1.0%, Nb: 0.005-0.04%, V: 0.01-0.10% andB: 0.0005-0.003%

[0053] Cr, Mo, Nb, V and B are all effective in increasing the strengthof steel and can be contained to produce such effect. However, excessivecontents thereof deteriorate the toughness and weldability. Thus, incases where they are caused to be contained in the steel, one or moremembers of the group consisting of these elements are caused to becontained desirably within the content range of not less than 0.05% butnot more than 1.0% in the case of Cr, not less than 0.05% but not morethan 1.0% in the case of Mo, not less than 0.005% but not more than0.04% in the case of Nb, not less than 0.01% but not more than 0.10% inthe case of V, and not less than 0.0005% but not more than 0.003% in thecase of B. For obtaining higher performance characteristics, thepreferred content ranges for the respective elements are as follows: Cr:0.1-0.3%, Mo: 0.1-0.3%, Nb: 0.005-0.02%, V: 0.01-0.05%, and B:0.0005-0.002%.

[0054] Ca: 0.0005-0.05%, Zr: 0.0005-0.05%, REMs: 0.0005-0.05%

[0055] Ca, Zr and REMs are effective in controlling the yield andmorphology of inclusions in steel and thus improving the toughness.Therefore, these may be caused to be contained in the steel forproducing this effect. However, excessive contents thereof may ratherimpair the toughness in certain instances. If these are caused to becontained, therefore, one or more of them are caused to be containeddesirably within the content range of not less than 0.0005% but not morethan 0.05% in the case of Ca, not less than 0.0005% but not more than0.05% in the case of Zr, and not less than 0.0005% but not more than0.05% in the case of REMs.

[0056] Other Elements

[0057] Those other elements than the elements mentioned above which aregenerally added in steel production may be added without any particularrestriction unless they weaken or nullify the effects of the invention.

[0058] Pcm: not more than 0.28

[0059] Pcm is an indicator of susceptibility to weld cracking. When thevalue of Pcm defined by the formula (1) given above is not more than0.28, no weld cracking occurs under ordinary welding conditions.Therefore, the value of Pcm should be not more than 0.28. Since when thePcm value is lower, the step of preheating can be omitted in carryingout welding, the Pcm value is desirably as small as possible.

[0060] When the steel contains B, the steel acquires an increased levelof hardenability. It is therefore desirable that the Pcm value berestricted according to the B content. For example, when the B contentis less than 0.0003%, it is desirable that the Pcm value be not morethan 0.21 and, when the B content is not lower than 0.0003%, the Pcmvalue is desirably not more than 0.19. By restricting the Pcm value inthat manner, it becomes possible to obtain good welding results evenwhen the welding is carried out under ordinary conditions withoutpreheating under environmental conditions of 25° C.

[0061] Steel Production Method 1:

[0062] The steel of the present invention can be produced by aproduction process comprising the following steps (a) to (e), which isgiven as an embodiment of the present invention:

[0063] Step (a): Heating a steel having the above chemical compositionto a temperature not lower than 950° C. but not higher than 1250° C.;

[0064] Step (b): Hot rolling the thus-heated steel;

[0065] Step (c): Allowing the hot-rolled steel to cool or cooling thesame in an accelerated manner;

[0066] Step (d): Reheating the steel after being allowed to cool oracceleratedly cooled to a temperature not lower than 450° C. but nothigher than 680° C.; and

[0067] Step (e): Air cooling the reheated steel.

[0068] In the following, the above steps (a) to (e) are described indetail.

[0069] Step (a)

[0070] A steel having the above chemical composition is heated to atemperature not lower than 950° C. but not higher than 1250° C. At aheating temperature below 950° C., it may become difficult in someinstances to secure the required finishing temperature in the next hotrolling step (b). At a heating temperature above 1250° C., on the otherhand, austenite grains coarsen and the toughness of the productdecreases.

[0071] Therefore, in the above embodiment of the invention, a heatingtemperature of not lower than 950° C. but not higher than 1250° C. isemployed for the heating of steel prior to hot rolling.

[0072] Step (b)

[0073] The hot rolling conditions may be the same as those used inproducing conventional steel products.

[0074] Step (c)

[0075] The steel after completion of the hot rolling is allowed to coolor acceleratedly cooled.

[0076] When, in this step, the cooling (allowing to cool oracceleratedly cooling) is carried out at a cooling rate of not slowerthan 1° C./sec, the formation of a bainite structure containing coarsecarbides can be suppressed and the strength can be increased to asatisfactory level even in the core of the steel product. Therefore,when the desired strength of the steel product is at a high level, it isdesirable in the above embodiment of the invention to employ a coolingrate of not slower than 1° C./sec in allowing the steel to cool orcooling the same acceleratedly.

[0077] On the other hand, when the cooling rate is in excess of 50°C./sec in allowing the steel to cool or cooling the same acceleratedly,the vicinity of the surface layer of the steel product may readily bequenched, so that the toughness of the surface layer of the steelproduct decreases in some instances. Therefore, in the above embodimentof the invention, it is desirable that the cooling rate in allowing thesteel to cool or cooling the same acceleratedly be not more than 50°C./sec.

[0078] Further, when the cooling is finished at a temperature above 580°C., the formation of martensite or lower bainite or the like becomesinsufficient not only in the core but also in the surface layer of thesteel and it becomes difficult in certain instances to secure thedesired strength. Therefore, when a high level of strength is requiredof the steel in the above embodiment of the invention, it is desirableto finish the allowing to cool or cooling acceleratedly at a temperaturenot higher than 580° C.

[0079] Step (d) and Step (e)

[0080] The steel cooled in step (c) is reheated to a temperature notlower than 450° C. but not higher than 680° C. and then air-cooled. Thisis for the purpose of efficiently and stably attaining Cu precipitationhardening. According to the desired strength and/or toughness, thereheating temperature is selected within the range of 450-680° C.

[0081] When, in step (d), the reheating temperature is lower than 450°C., the precipitation of Cu will not be fully finished. Therefore, incases where the intended steel strength is high, there arises thepossibility of such steel strength being not satisfactorily secured. Ata reheating temperature higher than 680° C., the steel strength cannotbe secured any longer due to over aging.

[0082] Therefore, in the above embodiment of the invention, thereheating temperature should be not lower than 450° C. but lower than680° C.

[0083] Steel Production Method 2:

[0084] The steel of the invention can also be produced by anotherprocess, which is another embodiment of the production process of thepresent invention and which comprises the following steps (A) to (C):

[0085] Step (A): Estimating, for a steel having the above chemicalcomposition, the change in tensile strength in the process ofstrain-removing heat treatment on the assumption that the steel may besubjected to strain-removing heat treatment after tempering undervarious conditions;

[0086] Step (B): Determining the tempering conditions based on thechange in tensile strength as estimated in step (A); and

[0087] Step (C): Tempering the steel under the tempering conditionsestablished in step (B).

[0088] In the following, the above steps (A) to (C) are described indetail.

[0089] Step (A)

[0090] For a steel having the above chemical composition, the change intensile strength in the process of strain-removing heat treatment isestimated on the assumption that the steel may be subjected tostrain-removing heat treatment after tempering under various conditions

[0091] The tensile strength of a Cu precipitation strengthened steelvaries in the process of strain-removing heat treatment thereof mainlydue to the change in the state of precipitation of Cu uponstrain-removing heat treatment and to the change in the matrix of steeland, therefore, can be calculated as a function of the strain-removingheat treatment temperature and the time.

[0092] Therefore, the estimation of the change in tensile strength inthe process of strain-removing heat treatment is desirably carried outbased on the estimation of Cu precipitation hardening in the process ofstrain-removing heat treatment, of Cu over age softening, and of tempersoftening of the steel matrix. In this case, in estimating the change intensile strength in the process of strain-removing heat treatment, it isdesirable to use a predictive equation involving the terms correspondingto Cu precipitation hardening in the process of strain-removing heattreatment, Cu over age softening and temper softening of the steelmatrix. The following equations (2) and (3), for instance, may be usedas the above predictive equation.

σ=M└B−exp(−P ₁ ^(1.5))+exp(−P ₂)+Cexp(−P ₃)┘  (2)

[0093] $\begin{matrix}{{P_{m} = {\sum\limits_{i}{A_{m}{{\exp \left( {- \frac{Q_{m}}{{RT}_{i}}} \right)} \cdot \Delta}\quad t_{i}}}}{{m = 1},2,3}} & (3)\end{matrix}$

[0094] In the above equations, σ is the tensile strength (MPa), R is thegas constant, Δt_(i) is each very short time interval, T_(i) is thesteel temperature (K) in that very short time interval.

[0095] A₁, A₂, A₃, Q₁, Q₂, Q₃, M, B and C are constants determined bythe chemical composition of steel and the production conditions, and canbe empirically determined by actually performing heat treatment of theCu-containing steel in question under various conditions and measuringthe tensile strength.

[0096] In carrying out the tensile strength estimation using theequations (2) and (3), the temperature change in the process ofstrain-removing heat treatment is approximated by a stairlike function,and the heat treatment time is divided into very short time intervals(e.g. 1 second) so that the steel temperature change during the intervalmay amount to not more than 10° C. and the i−th interval is representedby Δt_(i). When the steel temperature (absolute temperature) in the i−thtime interval is represented by T_(i) (K), the gas constant by R, theactivation energy by Q_(m) and the oscillation factor term by A_(m), thereaction rate constant is represented by A_(m)exp(−Q_(m)/RT_(i)).

[0097] Here, Q₁ is the activation energy for Cu precipitation, P₁ ^(1.5)is the degree of progress of age hardening, and −exp(−P₁ ^(1.5)) is aterm expressing the contribution of Cu precipitation to the tensilestrength.

[0098] Q₂ is the activation energy for coarsening of Cu precipitateparticles, P₂ is the degree of progress of over age softening due tocoarsening of Cu particles, and exp(−P₂) is a term expressing thesoftening in tensile strength due to coarsening of Cu precipitateparticles.

[0099] Q₃ is the activation energy relative to temper softening of thesteel matrix, P₃ is the degree of progress of softening due to temperingof the steel matrix, and Cexp(−P₃) is a term expressing the tempersoftening of the steel matrix.

[0100] While the constants appearing in the equations (2) and (3) mayvary depending on the chemical composition of the steel and theconditions of hot rolling and of the subsequent cooling, among others,the following values may be used, for example for a steel whose Cucontent is about 1% and whose tensile strength is about 480-650 MPa:Q₁=140 kJ/mol, Q₂=244 kJ/mol, Q₃=285 kJ/mol, A₁=5×10⁵, A₂=1×10¹¹,A₃=5×10¹¹, M=82.5 MPa, B=5.65, and C=1.

[0101] The anticipated strain-removing heat treatment may be a typicalheat treatment employed following welding and, for example, thefollowing strain-removing heat treatment conditions including the threesteps (I) to (IV) may be considered:

[0102] Step (I): Heating the steel from ordinary temperature to 580° C.at a heating rate of 55° C./hour;

[0103] Step (II): Maintaining the steel at 580° C. for 4 hours;

[0104] Step (III): Cooling the steel from 580° C. to 400° C. at acooling rate of 55° C./hour; and

[0105] Step (IV): Cooling the steel to room temperature at an arbitrarycooling rate.

[0106] The estimation of the change in tensile strength in the processof strain-removing heat treatment according to the equations (2) and (3)is recommendably carried out in the following manner. First, theequations (2) and (3) are stored in a computer. Then, various temperingand strain-removing heat treatment conditions are inputted into thecomputer. Then, for the cases where the various tempering andstrain-removing heat treatment conditions are applied, the change intensile strength at ordinary temperature is calculated by the computerat each time point during the period from the start of tempering to thecompletion of strain-removing heat treatment.

[0107]FIG. 1 (a) is a graphic representation of a heating pattern to befollowed on the assumption that a Cu-containing steel as rolled issubjected to strain-removing heat treatment. FIG. 1 (b) is a graphicrepresentation of the results of calculations for estimating the changein tensile strength at ordinary temperature as made for the sameCu-containing steel after application of the heating pattern shown inFIG. 1 (a)

[0108] As shown by the graph in FIG. 1 (b), the product as rolled has atensile strength of 549 MPa. At about 10 hours after the start of thestrain-removing heat treatment, the tensile strength is increased to 622MPa. Then, softening occurs due to over aging and, at the time offinishing of the strain-removing heat treatment, the tensile strengthbecomes 558 MPa. As a result, the change in tensile strength within therange of 549-622 MPa occurs in the vicinity of that site or sectionsubjected to the above strain-removing heat treatment.

[0109]FIG. 2 (a) is a graphic representation of a heating pattern to befollowed on the assumption that a Cu-containing steel is subjected totempering at 500° C. for 1 hour and strain-removing heat treatment. FIG.2 (b) is a graphic representation of the results of calculations forestimating the change in tensile strength at ordinary temperature asmade for the same Cu-containing steel after application of the heatingpattern shown in FIG. 2 (a).

[0110] As shown by the graph in FIG. 2 (b), tempering raises the tensilestrength to 581 MPa and, at about 10 hours after the start of the abovestrain-removing heat treatment, the tensile strength rises to 624 MPa,then softening occurs due to over aging and, at the time of completionof the strain-removing heat treatment, the tensile strength becomes 558MPa. As a result, the change in tensile strength within the range of558-624 MPa occurs in the vicinity of that section subjected to theabove strain-removing heat treatment.

[0111]FIG. 3 (a) is a graphic representation of a heating pattern to befollowed on the assumption that a Cu-containing steel is subjected totempering at 550° C. for 1 hour and strain-removing heat treatment. FIG.3 (b) is a graphic representation of the results of calculations forestimating the change in tensile strength at ordinary temperature asmade for the same Cu-containing steel after application of the heatingpattern shown in FIG. 3 (a).

[0112] As shown by the graph in FIG. 3 (b), the tensile strength at thetime of finishing of tempering is 620 MPa and, at the time of finishingof the strain-removing heat treatment, the tensile strength becomes 556MPa. As a result, the change in tensile strength within the range of556-620 MPa occurs in the vicinity of that section subjected to theabove strain-removing heat treatment.

[0113]FIG. 4 (a) is a graphic representation of a heating pattern to befollowed on the assumption that a Cu-containing steel is subjected totempering at 600° C. for 1 hour and strain-removing heat treatment. FIG.4 (b) is a graphic representation of the results of calculations forestimating the change in tensile strength at ordinary temperature asmade for the same Cu-containing steel after application of the heatingpattern shown in FIG. 4 (a).

[0114] As shown by the graph in FIG. 4 (b), the tensile strength at thetime of finishing of tempering is 581 MPa and, at the time of finishingof the strain-removing heat treatment, the tensile strength becomes 550MPa. As a result, the change in tensile strength within the range of550-581 MPa occurs in the vicinity of that section subjected to theabove strain-removing heat treatment.

[0115]FIG. 5 (a) is a graphic representation of a heating pattern to befollowed on the assumption that a Cu-containing steel is subjected totempering at 650° C for 1 hour and strain-removing heat treatment. FIG.5 (b) is a graphic representation of the results of calculations forestimating the change in tensile strength at ordinary temperature asmade for the same Cu-containing steel after application of the heatingpattern shown in FIG. 5 (a).

[0116] As shown by the graph in FIG. 5 (b), the change in tensilestrength within the range of 536-539 MPa occurs in the vicinity of thatsection subjected to the above strain-removing heat treatment.

[0117] Steps (B) and (C)

[0118] Based on the change in tensile strength in the strain-removingheat treatment as estimated in step (A), the tempering conditions areestablished and the steel is tempered under the tempering conditionsestablished.

[0119] In the above mode of embodiment of the present invention, it isintended that when the steel of the invention is used as a material forconstructing a welded structure, the stability of the welded structurebe improved by reducing the change in steel material strength as causedby strain-removing heat treatment following welding.

[0120] Therefore, it is desirable to determine the tempering conditionsso that the change in tensile strength as estimated in step (A) may benot more than 50 MPa. The reason why the tempering conditions aredetermined based on the change in tensile strength in the process ofstrain-removing heat treatment, not on the change in tensile strengthbetween the value before and the value after the strain-removing heattreatment is as follows.

[0121] From the welded structure stability viewpoint, not only thechange in tensile strength in the section subjected to strain-removingheat treatment but also the change in tensile strength at sites awayfrom the section subjected to strain-removing heat treatment is to betaken into consideration. The sites away from the section subjected tostrain-removing heat treatment undergo shorter periods of heat treatmentat lower temperatures as compared with the section subjected tostrain-removing heat treatment, so that the mechanical characteristicsof those sites correspond to the mechanical characteristics which thesection subjected to strain-removing heat treatment shows during theheat treatment. Therefore, it is necessary to determine the temperingconditions based not only on the change in tensile strength between thevalue before and the value after strain-removing heat treatment but alsoon the change in tensile strength in the process of strain-removing heattreatment.

[0122] Here, the tempering conditions may be arbitrary ones providedthat the desired strength can be secured thereunder. Generally, theheating rate is 400-2000° C./hour, the heating temperature is 400-700°C., and the cooling rate is 100-10000° C./hour, for instance.

EMBODIMENTS First Embodiment

[0123] Cu precipitation strengthened steel plates were producedaccording to the chemical composition and production conditionsspecified in Table 1, and test specimens were cut out of the steelplates and measured for YS, TS, and critical CTOD value. The CTOD testwas performed according to the method of BS 7448; the test temperaturewas −40° C. The results are summarized in Table 1. TABLE 1 Test Chemicalcomposition (mass %: Fe and other incidental impurities) No. C Si Mn P SCu Ni Nb Ti Se Al N Pcm  1 0.06 0.19 1.3 0.013 0.006 1.05 0.5 0.01 0.0120.001 0.01 0.004 0.19  2 0.06 0.19 1.5 0.011 0.006 1.05 0.5 0.01 0.0120.001 0.01 0.004 0.20  3 0.05 0.19 1.4 0.012 0.006 1.05 0.3 0.01 0.0120.001 0.01 0.004 0.18  4 0.05 0.18 1.4 0.012 0.006 1.05 0.4 0.01 0.0120.001 0.01 0.004 0.19  5 0.02 0.18 2.0 0.012 0.006 1.05 0.7 0.02 0.0120.001 0.01 0.004 0.19  6 0.10 0.06 1.0 0.012 0.006 1.05 0.4 0.01 0.0120.001 0.01 0.004 0.21  7 0.04 0.06 1.6 0.012 0.006 1.05 0.4 0.01 0.0120.001 0.01 0.004 0.18  8 0.07 0.07 0.05 0.012 0.006 1.05 0.4 0.01 0.0120.001 0.01 0.004 0.16  9 0.05 0.06 1.4 0.025 0.006 1.05 0.4 0.01 0.0120.001 0.01 0.004 0.18 10 0.05 0.06 1.4 0.013 0.01 1.05 0.4 0.01 0.0120.001 0.01 0.004 0.18 11 0.08 0.12 1.2 0.013 0.006 0.50 0.4 0.01 0.0120.001 0.01 0.003 0.18 12 0.05 0.11 1.2 0.013 0.006 2.00 0.4 0.01 0.0120.001 0.01 0.003 0.22 13 0.05 0.11 1.4 0.013 0.006 1.05 3.5 0.01 0.0120.001 0.01 0.003 0.23 14 0.05 0.11 1.4 0.013 0.006 1.05 0.4 0.005 0.0120.001 0.01 0.003 0.18 15 0.05 0.11 1.4 0.013 0.006 1.05 0.4 0.02 0.0050.001 0.01 0.003 0.18 16 0.05 0.11 1.4 0.013 0.006 1.05 0.4 0.01 0.0120.001 0.01 0.003 0.18 17 0.05 0.12 1.5 0.013 0.006 1.05 0.4 0.01 0.0120.001 0.01 0.003 0.19 18 0.05 0.11 1.4 0.013 0.006 1.05 0.4 0.01 0.0120.005 0.01 0.003 0.18 19 0.09 0.12 1.5 0.017 0.007 1.01 0.5 0.01 0.0120.002 0.01 0.003 0.23 20 0.05 0.11 1.4 0.012 0.007 1.01 0.5 0.01 0.0120.002 0.01 0.003 0.18 21 0.05 0.11 1.4 0.012 0.007 1.01 0.5 0.01 0.0120.002 0.01 0.003 0.18 22 0.03 0.12 0.8 0.015 0.006 0.50 0.5 0.005 0.0120.002 0.01 0.003 0.11 23 0.05 0.11 1.4 0.012 0.005 1.00 0.5 0.01 0.0120.002 0.01 0.003 0.18 24 0.01 0.12 1.5 0.012 0.007 1.01 0.5 0.01 0.0120.002 0.01 0.003 0.15 25 0.12 0.11 1.0 0.012 0.007 1.01 0.5 0.01 0.0120.002 0.01 0.003 0.23 26 0.05 0.11 1.4 0.03 0.007 1.01 0.5 0.01 0.0120.002 0.01 0.003 0.18 27 0.05 0.11 1.4 0.012 0.02 1.01 0.5 0.01 0.0120.002 0.01 0.003 0.18 28 0.08 0.11 1.2 0.012 0.008 0.40 0.5 0.01 0.0120.002 0.01 0.003 0.17 29 0.08 0.12 1.7 0.021 0.009 2.35 0 0.03 0.0120.003 0.008 0.003 0.29 30 0.04 0.12 1.1 0.012 0.005 0.80 0.5 0.003 0.080.002 0.007 0.003 0.15 31 0.08 0.11 1.8 0.018 0.008 1.70 0.2 0.05 0.0120.004 0.2 0.003 0.26 32 0.08 0.11 1.6 0.02 0.008 1.15 0.5 0.06 0.0120.006 0.014 0.003 0.23 Production conditions Product characteristicsHeating Cooling Cooling Reheating Minimum critical Test temp. ratefinish temp. temp. YS TS CTOD value No. (° C.) (° C./sec) (° C.) (° C.)(MPa) (MPa) (mm)  1 1250 Allowed to cool Allowed to cool 600 590 6801.25  2  950 Allowed to cool Allowed to cool 600 572 660 1.41  3 1050 15580 600 569 639 1.34  4 1050 50 550 600 560 682 1.35  5 1050 18 550 600571 660 1.42  6 1050 16 550 600 585 669 1.32  7 1050 15 550 600 570 6501.32  8 1050 18 550 600 563 651 1.32  9 1050 15 550 600 555 644 1.34 101050 18 550 600 559 651 1.33 11 1050 15 550 600 555 654 1.41 12 1050 16550 600 587 673 1.33 13 1050 15 550 600 570 650 1.43 14 1050 18 550 600568 649 1.32 15 1050 18 550 600 570 650 1.32 16 1050 15 550 450 590 6721.28 17 1050 15 550 680 559 645 1.43 18 1050 15 550 600 559 645 0.82 191300 Allowed to cool Allowed to cool 580 590 690 0.76 20  900 Allowed tocool Allowed to cool 500 530 605 1.21 21 1100 18 620 580 530 645 0.75 221240 12 570 400 532 625 0.73 23  950 15 560 700 515 609 1.23 24 1050 15550 580 533 620 1.21 25 1100 18 550 580 538 610 0.76 26 1100 16 550 580535 640 0.74 27 1100 15 550 580 540 652 0.74 28 1100 18 550 580 480 5801.22 29 1240 18 450 550 710 820 0.31 30 1100 15 550 580 510 612 0.72 311220 18 480 560 700 815 0.33 32 1200 20 400 550 620 715 0.015

[0124] In Test No. 32, in which the Se content was above the upper limitprescribed according to the invention, the minimum critical CTOD valuewas markedly lower as compared with other test materials.

[0125] In Test Nos. 24-31, in which the Se content was within the rangeprescribed by the invention but the content of some other element thanSe was outside the range prescribed by the invention, the CTOD toughnessor strength was poor.

[0126] In Test Nos. 19-23, in which the chemical composition was withinthe range specified by the invention but the production conditions werenot appropriate, the CTOD toughness or strength was slightly inferior.

[0127] In Test Nos. 1-18, in which the chemical composition and Pcmsatisfied the requirements of the present invention and the productionconditions were appropriate, the strength was high and the CTODtoughness was good.

Second Embodiment

[0128] Steels having the composition specified in Table 2 were melted ina converter and 300-mm-thick slabs were produced therefrom by continuouscasting. TABLE 2 Chemical composition (mass %; balance: Fe and otherincidental impurities) Steel C Si Mn P S Cu Ni Cr Mo Nb V Ti Al N SeOther Pcm A 0.04 0.09 1.42 0.007 0.001 0.99 0.48 0.08 0.22 0.009 — 0.0110.006 0.0038 <0.001 — 0.19 B 0.04 0.05 1.95 0.005 0.001 1.12 0.62 — — —— 0.028 0.012 0.0028 <0.001 — 0.21 C 0.03 — 0.92 0.019 0.001 1.01 0.610.97 0.12 0.012 — 0.013 0.021 0.0025 <0.001 — 0.19 D 0.07 0.23 0.310.011 0.004 1.05 0.75 0.12 0.22 0.039 0.051 0.004 0.023 0.0036 <0.001Ca:0.0026 0.18 E 0.04 0.11 1.51 0.009 0.002 0.98 1.01 0.21 0.21 0.011 —0.011 0.005 0.0033 <0.001 — 0.21 F 0.04 0.08 0.86 0.008 0.001 0.65 3.960.13 0.12 0.023 0.046 0.014 0.096 0.0039 <0.001 — 0.20 G 0.05 0.13 0.820.009 0.002 1.93 1.03 — — — — 0.008 — 0.0056 <0.001 — 0.21 H 0.04 0.150.86 0.006 0.001 0.85 0.51 — 0.96 — — 0.015 0.013 0.0012 <0.001 — 0.20 I0.03 0.12 0.35 0.011 0.001 0.21 0.32 0.11 0.23 — 0.041 0.012 0.0110.0034 <0.001 — 0.09 J 0.04 0.06 0.31 0.012 0.002 2.98 0.11 — — 0.012 —0.011 0.014 0.0022 <0.001 — 0.21 K 0.03 0.06 0.54 0.011 0.001 0.61 0.31— 1.8 0.018 — 0.009 0.011 0.0027 <0.001 — 0.21 L 0.12 0.36 1.89 0.0090.002 1.11 0.65 0.11 0.13 0.021 0.038 0.021 0.023 0.0031 <0.001 — 0.31 M0.06 0.23 1.42 0.009 0.001 1.02 0.56 — 0.12 0.025 — 0.013 0.012 0.00230.026 — 0.21

[0129] The slabs were heated to 1150° C. and hot-rolled under variousconditions shown in Table 3 with reduction rates of not less than 2 at900° C. or below, and then cooled to give steel plates having a width of2000 mm and varying in thickness.

[0130] The steel plates as rolled were measured for tensile strength.Further, for each steel, the ordinary temperature tensile strengthvalues to be obtained upon various tempering procedures andstrain-removing heat treatment comprising the above steps (I) to (IV)were estimated by calculation using the above equations (2) and (3), andthe absolute values (estimated values) of the changes in tensilestrength in the process of the above strain-removing heat treatment werecalculated.

[0131] The constants in the equations (2) and (3) were given thefollowing values: Q₁=140 kJ/mol, Q₂=244 kJ/mol, Q₃=285 kJ/mol, M=82.5MPa, A₁ =5×10⁵, A₂ =1×10¹¹, A₃=5×10¹¹, B=5.65, and C=1.

[0132] The above as-rolled steel plates were tempered by heating to oneof temperatures at 50° C. intervals within the range of not lower than500° C. to not higher than 650° C. over a temperature raising time of 1hour, maintaining at that temperature for 1 hour and then cooling toordinary temperature over a temperature lowering time of 1 hour, to giveproduct steel plates.

[0133] A large number of test specimens were cut out from each productsteel plate, and subjected to heat treatment by heating from ordinarytemperature to 580° C. at a rate of 55° C./hour, maintaining at thattemperature for a period selected at 15-minute intervals within therange of not short than 15 minutes but not longer than 4 hours andcooling rapidly to room temperature.

[0134] Steel plates subjected to the above-mentioned strain-removingheat treatment were also prepared by raising the temperature fromordinary temperature to 580° C., maintaining at 580° C. for 4 hours,then lowering the temperature from 580° C. to 400° C. at a rate of 55°C./hour and thereafter rapidly cooling to room temperature.

[0135] No. 4 tensile test specimens prescribed in JIS Z 2201 were takenwith the plate width direction as the longitudinal direction ofspecimens from middle portion, with respect to the plate thickness, ofeach steel plate after the above heat treatment and measured for tensilestrength at ordinary temperature and, in this way, the absolute values(actually measure values) of the changes in tensile strength in theprocess of the above strain-removing heat treatment were examined.

[0136] The hot workability was evaluated based on the occurrence ornonoccurrence of surface cracking as judged by visual observation of thesurface of each tempered product steel plate.

[0137] Further, No. 4 tensile test specimens prescribed in JIS Z 2201with the plate width direction taken as the longitudinal directionthereof as well as No. 4 Charpy test specimens prescribed in JIS Z 2202were taken from the middle portion, with respect to the plate thickness,of each steel plate and examined for tensile test characteristics andimpact test characteristics.

[0138] Further, a y-groove weld cracking test was conducted according toJIS Z 3158 to evaluate each steel plate for susceptibility to weldcracking. The weld cracking test was carried out in an atmosphere at atemperature of 25° C. and a humidity of 60% at a test specimen initialtemperature of 25° C.

[0139] The results of various tests are summarized in Table 3. TABLE 3Change in ten- Change in ten- sile strength Surface sile strength Hotrolling Cooling after hot rolling upon strain- crack- upon strain-Finish Cooling Tem- removing heat ing dur- Product steel plate removingheat Finish thick- Initial rate *1 Finish pering treatment ing hotcharacteristics treatment (act- y Test temp. ness temp (° C./ temp.temp. (estimated) working YS TS vE-40 ually meas- crack- No. Steel (°C.) (° C.) (° C.) sec) (° C.) (° C.) (MPa) *2 (MPa) (MPa) (J) ured)(MPa) ing *2 51 A 820 100 770 3.8 340 600 31 ∘ 542 620 294 36 602 52 A820 100 770 3.7 340 650  3 ∘ 545 621 294 13 ∘ 53 B 770 100 730 3.7 350600 31 ∘ 599 662 294 26 ∘ 54 C 820  75 770 4.8 350 600 31 ∘ 650 682 10229 ∘ 55 D 780  50 760 7.2 480 600 31 ∘ 540 629 294 33 ∘ 56 E 780  50 7607.2 480 600 31 ∘ 637 694 256 25 ∘ 57 F 820  75 770 4.9 360 600 31 ∘ 674733 294 10 ∘ 58 G 790  50 760 7.3 490 650  3 ∘ 501 594 282  5 ∘ 59 H 760100 720 3.5 330 650  3 ∘ 833 860 152  4 ∘ 60 I 770 100 730 3.7 340 650 3 ∘ 442 470 235  6 ∘ 61 J 780  50 760 7.3 490 650  3 x 589 659 168  4 ∘62 K 780  50 750 7.1 470 600 31 ∘ 821 856  23 22 ∘ 63 L 820  25 — AC —600 31 ∘ 543 576 294 28 x 64 M 820  75 770 4.8 360 650  3 ∘ 586 664  41 6 ∘ 65 A 770 100 — AC — — 73 ∘ 356 516 244 82 ∘ 66 A 820 100 770 3.6490 500 66 ∘ 550 639 161 63 ∘ 67 A 770 100 — AC — 550 65 ∘ 494 599 24558 ∘

[0140] As shown in Table 3, the steel plates of Test Nos. 51-59 showedno surface crack and had a tensile strength of not lower than 480 MPaand a Charpy absorbed energy of not less than 50 J at −40° C. and thusshowed good characteristics such that no cracking occurred in they-groove weld cracking test. They each also showed good characteristicsas evidenced by a change (actually measured value) in tensile strengthof not greater than 50 MPa (in absolute value) during the abovestrain-removing heat treatment and by stable material characteristicsafter welding. The estimated values and actually measured values ofchanges in tensile strength in the process of the above strain-removingheat treatment were in good agreement with each other and, thus, theprecision of tensile strength estimation by the equations (2) and (3)was good.

[0141] On the contrary, in Test No. 60, in which steel I insufficient inCu content was used, the steel was insufficient in strength. In Test No.61, in which steel J excessive in Cu content was used, surface crackswere formed during hot rolling. In Test No. 62, in which steel Kexcessive in Mo content was used, the toughness of the product steelplate was insufficient. In Test No. 63, in which steel L having anexcessively high Pcm value was used, cracks appeared in the y-grooveweld cracking test and, in Test No. 64, in which steel M containing alarge amount of the incidental impurity Se was used, the toughness ofthe product steel plate was poor.

[0142] With steel A, the change in tensile strength of the product steelplate in the process of the above strain-removing heat treatment was notmore than 50 MPa and the stability of the material characteristicsagainst heat treatment after welding was good in Test No. 51, in whichthe tempering temperature was 600° C., and in Test No. 52, in which thetempering temperature was 650° C., but, in Test No. 65, in which notempering was performed, and in Test Nos. 66 and 67, in which thetempering temperature was 500° C. or 550° C., the tensile strengthchange in the process of the above strain-removing heat treatmentexceeded 50 MPa, hence the stability was not good.

What is claimed is:
 1. A Cu precipitation strengthened steel whichcomprises, on the mass percent basis, C: 0.02-0.10%, Mn: 0.3-2.5%, Cu:0.5-2.0%, Ni: 0.3-4.0% and Ti: 0.004-0.03% and further comprises Si:0.01-0.4% and/or Al: 0.001-0.1%, with the contents of incidentalimpurities being P: not more than 0.025%, S: not more than 0.01%, N: notmore than 0.006% and Se: not more than 0.005%, with the value of Pcmdefined by the formula (1) given below being not more than 0.28:$\begin{matrix}{{Pcm} = {C + \frac{Si}{30} + \frac{Mn}{20} + \frac{Cu}{20} + \frac{Ni}{60} + \frac{Cr}{20} + \frac{Mo}{15} + \frac{V}{10} + {5B}}} & (1)\end{matrix}$

where C, Si, Mn, Cu, Ni, Cr, Mo, V and B respectively represent thecontents (in mass percent) of the respective elements.
 2. A Cuprecipitation strengthened steel as claimed in claim 1 which furthercomprises, on the mass percent basis, at least one element selected fromthe group consisting of Cr: 0.05-1.0%, Mo: 0.05-1.0%, Nb: 0.005-0.04%,V: 0.01-0.10% and B: 0.0005-0.003%.
 3. A Cu precipitation strengthenedsteel as claimed in claim 1 which further comprises, on the mass percentbasis, at least one element selected from the group consisting of Ca:0.0005-0.05%, Zr: 0.0005-0.05% and REMs: 0.0005-0.05%.
 4. A Cuprecipitation strengthened steel as claimed in claim 2 which furthercomprises, on the mass percent basis, at least one element selected fromthe group consisting of Ca: 0.0005-0.05%, Zr: 0.0005-0.05% and REMs:0.0005-0.05%.
 5. A Cu precipitation strengthened steel as claimed inclaim 1, wherein the content of Se is not more than 0.001%.
 6. A Cuprecipitation strengthened steel as claimed in claim 5 which furthercomprises, on the mass percent basis, at least one element selected fromthe group consisting of Cr: 0.05-1.0%, Mo: 0.05-1.0%, Nb: 0.005-0.04%,V: 0.01-0.10% and B: 0.0005-0.003%.
 7. A Cu precipitation strengthenedsteel as claimed in claim 5 which further comprises, on the mass percentbasis, at least one element selected from the group consisting of Ca:0.0005-0.05%, Zr: 0.0005-0.05% and REMs: 0.0005-0.05%.
 8. A Cuprecipitation strengthened steel as claimed in claim 6 which furthercomprises, on the mass percent basis, at least one element selected fromthe group consisting of Ca: 0.0005-0.05%, Zr: 0.0005-0.05% and REMs:0.0005-0.05%.
 9. A process for producing a Cu precipitation strengthenedsteel which comprises the following steps: Step (a): Heating a steelcomprising, on the mass percent basis, C: 0.02-0.10%, Mn: 0.3-2.5%, Cu:0.5-2.0%, Ni: 0.3-4.0% and Ti: 0.004-0.03% and further comprising Si:0.01-0.4% and/or Al: 0.001-0.1%, with the contents of incidentalimpurities being P: not more than 0.025%, S: not more than 0.01%, N: notmore than 0.006% and Se: not more than 0.005%, with the value of Pcmdefined by the formula (1) given below being not more than 0.28 to atemperature of 950-1250° C.; Step (b): Hot rolling the thus-heatedsteel; Step (c): Allowing the hot-rolled steel to cool or cooling thesame in an accelerated manner; Step (d): Reheating the steel after beingallowed to cool or acceleratedly cooled to a temperature of 450-680° C.;and Step (e): Air cooling the reheated steel. $\begin{matrix}{{Pcm} = {C + \frac{Si}{30} + \frac{Mn}{20} + \frac{Cu}{20} + \frac{Ni}{60} + \frac{Cr}{20} + \frac{Mo}{15} + \frac{V}{10} + {5B}}} & (1)\end{matrix}$


10. A process for producing a Cu precipitation strengthened steel asclaimed in claim 9, wherein, in step (c), the rate of cooling of thesteel by being allowed to cool or acceleratedly cooled is 1-50° C./s.11. A process for producing a Cu precipitation strengthened steel asclaimed in claim 9, wherein, in step (a), the steel further comprises,on the mass percent basis, at least one element selected from the groupconsisting of Cr: 0.05-1.0%, Mo: 0.05-1.0%, Nb: 0.005-0.04%, V:0.01-0.10% and B: 0.0005-0.003%.
 12. A process for producing a Cuprecipitation strengthened steel as claimed in claim 9, wherein, in step(a), the steel further comprises, on the mass percent basis, at leastone element selected from the group consisting of Ca: 0.0005-0.05%, Zr:0.0005-0.05% and-REMs: 0.0005-0.05%.
 13. A process for producing a Cuprecipitation strengthened steel as claimed in claim 11, wherein, instep (a), the steel further comprises, on the mass percent basis, atleast one element selected from the group consisting of Ca:0.0005-0.05%, Zr: 0.0005-0.05% and REMs: 0.0005-0.05%.
 14. A process forproducing a Cu precipitation strengthened steel which comprises thefollowing steps: Step (A): Estimating, for a steel comprising, on themass percent basis, C: 0.02-0.10%, Mn: 0.3-2.5%, Cu: 0.5-2.0%, Ni:0.3-4.0% and Ti: 0.004-0.03% and further comprising Si: 0.01-0.4% and/orAl: 0.001-0.1%, with the contents of incidental impurities being P: notmore than 0.025%, S: not more than 0.01%, N: not more than 0.006% andSe: not more than 0.005%, with the value of Pcm defined by the formula(1) given below being not more than 0.28, the change in tensile strengthin the process of strain-removing heat treatment on the assumption thatthe steel may be subjected to strain-removing heat treatment aftertempering under various conditions; Step (B): Determining the temperingconditions based on the change in tensile strength as estimated in step(A); and Step (C): Tempering the steel under the tempering conditionsestablished in step (B). $\begin{matrix}{{Pcm} = {C + \frac{Si}{30} + \frac{Mn}{20} + \frac{Cu}{20} + \frac{Ni}{60} + \frac{Cr}{20} + \frac{Mo}{15} + \frac{V}{10} + {5B}}} & (1)\end{matrix}$


15. A process for producing a Cu precipitation strengthened steel asclaimed in claim 14, wherein, in step (B), the tempering conditions areselected so that the change in tensile strength as estimated may amountto not more than 50 MPa.
 16. A process for producing a Cu precipitationstrengthened steel as claimed in claim 14, wherein, in step (A), theestimation is made based on the estimation of Cu precipitationstrengthening in the process of strain-removing heat treatment, of Cuover age softening, and of temper softening of the steel matrix.
 17. Aprocess for producing a Cu precipitation strengthening steel as claimedin claim 16, wherein, in step (A), the estimation of Cu precipitationstrengthened in the process of strain-removing heat treatment, theestimation of Cu over age softening and the estimation of tempersoftening of the steel matrix are made utilizing the following equations(2) and (3): σ=M└B=exp(−P ₁ ^(1.5))+exp(−P ₂)+Cexp(—P ₃)┘(2)$\begin{matrix}{{P_{m} = {\sum\limits_{i}{A_{m}{{\exp \left( {- \frac{Q_{m}}{{RT}_{i}}} \right)} \cdot \Delta}\quad t_{i}}}}{{m = 1},2,3}} & (3)\end{matrix}$


18. A process for producing a Cu precipitation strengthened steel asclaimed in claim 14, wherein, in step (A), the steel further comprises,on the mass percent basis, at least one element selected from the groupconsisting of Cr: 0.05-1.0%, Mo: 0.05-1.0%, Nb: 0.005-0.04%, V:0.01-0.10% and B: 0.0005-0.003%.
 19. A process for producing a Cuprecipitation strengthened steel as claimed in claim 14, wherein, instep (A), the steel further comprises, on the mass percent basis, atleast one element selected from the group consisting of Ca:0.0005-0.05%, Zr: 0.0005-0.05% and REMs: 0.0005-0.05%.
 20. A process forproducing a Cu precipitation strengthened steel as claimed in claim 18,wherein, in step (A), the steel further comprises, on the mass percentbasis, at least one element selected from the group consisting of Ca:0.0005-0.05%, Zr: 0.0005-0.05% and REMs: 0.0005-0.05%.